Hi Donna,
I can speak to this, to a degree, because of my experience with my girlfriend who had a adverse reaction from Asacol. Her reaction got progressively worse over a period of about 6 months. Her reaction was that the asacol had the opposite effect and made her D worse, with progressively worsening blood incidents, and eventually it started shutting down her kidneys. This was scary when that started happening.
I was upset with her GI because he increased her dose so high. But when she phoned him, and he listened to her symptoms, he told her to stop altogether for 3 days to see if the symptoms improve. Which they did imptove. He then discontinued the asacol, and he also assumed she would have the same reaction to all 5-ASA compunds. The research, however, indicates that other forms might not cause adverse reactions, whereas Asacol might have.
There are several formulations of 5-ASA which is (in laymens terms) is a form of analgesic to reduce inflammation similar in effect to asprin. If you can't tolerate asprin, you probably won't be able to tolerate any of the 5-ASA drugs. As folks have mentioned, the primary differences in the 5-ASA categories are the method the active drug is delivered and/or the location of where the active drug is released in the digestive tract. The different formulations of 5-ASA can be effective for one person and another person might have an adverse reaction.
Asacol and Pentasa are the same formulation of 5-ASA. However their release is substantially different. Asacol is released based on pH (acid) levels in the gut and Pentasa is released based on time-release. Research indicates that there is a slightly higher level of adverse renal (kidney) insufficiency with Asacol and Pentasa compared to the other variations of 5-ASA.
The 5-ASA compound with the highest level of intolerance is Sulfasalazine because it uses a sulfa agent to deliver the 5-ASA, and the reaction is mostly against the sulfa agent rather than the 5-ASA.
Back to my girlfreind's reaction, it seemed the biggest problem with 5-ASA (Asacol in her case) is you have to take such a large dosage many times a day. The drug also has some harsh side-effects. Also, it seems with the dosage so high over time, we're almost destined to develop an adverse reaction even if it didn't happen at first.